Is a layperson obligated to act upon the fatwa of the most reliable scholar, or by inquiring about the most preponderant opinion and acting upon it, especially since "the most preponderant opinion" is closer to what is correct when scholars differ?

Preponderance is a relative matter; what one scholar considers preponderant, another may not. When scholars differ, it is incumbent upon the layperson to follow the one whose knowledge and piety they trust, and this is precisely what is meant by following the preponderant opinion. It is permissible to follow the opinion of a deceased scholar just as it is permissible to follow that of a living one, because opinions do not die with the death of their proponents.